1. Field of the Invention
This invention relates to a polycrystalline silicon rod for preparing a monocrystalline silicon rod (or a single crystalline silicon rod) by a floating zone melting method (or a floating zone method) and a process for making the polycrystalline silicon rod, and particularly to the same for preparing the monocrystalline silicon rod for a semiconductor by the single floating zone method and the process for making the same.
2. Description of the Prior Art
The floating zone method has been developed by paying attention to properties of silicon that its density is relatively low and its surface tension is high, wherein a molten portion of silicon is lain between an unmelted portion of the polycrystalline silicon rod and a solidified portion thereof, a part of the molten portion is kept in a floating state (such molten portion is hereinafter referred to as a floating molten zone) and monocrystalline silicon is grown from a melt which is obtained by melting the polycrystalline silicon rod, without being brought into contact with a container to support the melt and any other materials.
In a vertical floating zone method, for example, Japanese Patent Publication No. 56-39048 describes that, with respect to a polycrystalline silicon rod with a seed crystal of monocrystalline silicon being attached to the lower end and which is vertically held, a boundary surface between the seed crystal and the polycrystal line rod is heated and molten by high frequency induction heating, electron beam heating, radiation heating or the like to form the floating molten zone and the thus molten portion is crystallized and the single crystal is grown, while gradually moving the above seed crystal portion downward with rotating and enlarging the size of said floating molten zone.
However, in the case of preparing the monocrystalline silicon rod from the polycrystalline silicon rod by the above floating zone method, the crystallization according to single passage of the floating molten zone does not result in satisfactory monocrystallization, and therefore the monocrystalline silicon rod is prepared through crystallization of the polycrystalline silicon rod in which at least two passages of the floating molten zone are carried out, in said first passage the polycrystalline silicon bar being in part monocrystallized.
Preparation of the single crystal by carrying out the at least two passages of the floating molten zone as mentioned above becomes a problem awaiting solution because much labor and time are necessary.